One aspect I'm interested in with the tilt shift is the ability to create stitched shots by just shifting the lens.
A tilt shift lens provides a much bigger image circle than a normal lens and shift moves the lens across that image circle, due to low distortion and no real camera movements (though more on that later) the stitching can be quite easy.
The downside is that you are moving towards the edge of the lens. This means lower IQ at the borders, vignetting and its more susceptible to show any filters used.
This morning I decided to have a play see what you can achieve. All shots taken on a 5d2.This is a standard unshifted frame (well 3, hdr'd in photomatix) at 24mm. Image size is about 5600x3700.
Shifting that setup to the maximum left and right (this is in reality 9 shots bracketed from -12mm,0mm,+12mm and -2,0,+2), this image is 9400x3700. Equivalent to about 14mm wide but tons more resolution, downside is the inability to shoot it in one frame so moving subjects are mostly out. Because I didnt shift the camera as well I have parallax issues with some stuff near to the camera on the right, technically I think I need to shift the camera in the opposite direction to the lens shift (keeping the lens in effectively the same place for each frame)
This shot is the camera in portrait orientation shifted horizontally. size is 7500x5700. Better edge definition than the landscape shift above
This is portrait mode shifted in vertical plane , 3700x9200.
Overall impressed with the ability to turn a 24mm lens into something much wider, I could easily shift like the last one above and also rotate to create a much wider panoramic with some huge dimensions. I'll certainly have to learn the right trick of shifting the camera at the same time to avoid parallax issues. The low distortion makes stitching with 24mm OK, I still think normally your better off with a longer focal length (50mm on FF seems ideal).